Show ID & description of commit that failed to merge during rebase

The idea is to give the person trying to solve the conflict as much information as possible. It's true that the failed merges contain that information, but it's located in hard to reach places: inside the conflicting files (and not in the status buffer), at the end of a conflict (and not at the beginning), and for binary files that information is not available at all.

That information was also available from looking at the `*magit-process*` buffer, but that buffer is erased with a lot of operations -- especially (un)staging.